Owing to their low weight, titanium materials are particularly suitable for the production of components for vehicle construction. However, use of conventional titanium alloys in the region of the exhaust gas system of internal-combustion engines is fundamentally opposed by the fact that in the case of heating to temperature above 600° C. they have a risk of fracture as a consequence of coarse grain formation. Coarse grain formation occurs in particular if the titanium material is exposed to a high operating temperature for a relatively long time.
Attempts have been made to alleviate the problem of coarse grain formation by alloying titanium with Fe or Si. These elements each have a grain-refining effect. However their drawback lies in the fact that as the Si and Fe contents increase, the ductility of the titanium material decreases so drastically that it can no longer be economically shaped. This property of titanium materials alloyed with Fe and Si has proven to be particularly disadvantageous if components with complex shapes are to be produced therefrom for the exhaust gas system of an internal-combustion engine.
A further possibility of reducing the risk of embrittlement as a result of oxidation of a titanium material is known from DE 101 03 169 A1. According to the method described in this document a titanium sheet is provided with an Al covering layer by roll-bonding and heat-treatment, which covering layer protects the Ti sheet from oxidation. However, the Al covering layer cannot prevent coarse grain formation in the Ti base material that occurs as a consequence of high-temperature heating that lasts a relatively long time.
In addition to the above-described prior art a Ti alloy is known from JP 2001 089821 A which contains (in % by weight) more than 1% and up to 5% Fe, 0.05 to 0.75% O and furthermore 0.01*e(0.5*% Fe) to 0.5*e−(0.5*% Fe)% Si, where % Fe represents the respective Fe content. The thus composed Ti alloy is intended not only to have high strength and be very ductile but should also have excellent resistance to oxidation at high temperatures. In practice however it has been found that even Ti alloys composed in this manner cannot permanently withstand the operating temperatures which occur for example in the region of exhaust gas systems of motor vehicles.
A Ti alloy for biological applications is also known from JP 04 105659 A, in particular for the production of artificial bones, which in addition to silicon contents should also contain yttrium contents to optimise compatibility of the alloy with the body. When processing this known Ti alloy a heat treatment is carried out which causes a near-surface layer to form in which Si and Y are present in enriched form. This document does not address the problem of high-temperature resistance of a Ti alloy, however.